1. Field of the Invention
The present invention relates to a multilayer substrate used for production of devices and such, and particularly a multilayer substrate having a diamond film.
2. Description of the Related Art
Diamond has a wide band gap of 5.47 eV and a very high dielectric breakdown electric field intensity of 10 MV/cm. Furthermore, it has the highest thermal conductivity in materials. Therefore, if this is used for an electric device, the device is advantageous as a high output power device.
Furthermore, diamond has high drift mobility and is the most advantageous as a high-speed power device in semiconductors in comparison of Johnson performance index.
Accordingly, diamond is said to be the ultimate semiconductor suitable for high frequency/high power electric devices.
Therefore, a multilayer substrate in which a diamond film and the like is laminated on a substrate has attracted attention.
Now, most of the single crystal diamonds for producing diamond semiconductors are referred to as Ib type formed by high-pressure method. These Ib-type diamonds contain much impurity of nitrogen and can only be obtained at a size of no more than about a 5-mm square and are low in utility.
By contrast, Chemical Vapor Deposition (CVD) method has an advantage that a diamond film of polycrystalline diamond of a large area having a diameter of about 6 inches (150 mm) can be obtained with high purity. However, in the CVD method, it has been conventionally difficult to perform single crystallization suitable for general electric devices. This is caused by a single crystal Si having been used as a substrate conventionally. That is, this is because Si and diamond are very different in lattice constant (mismatch between them is 52.6%) and it is very difficult to heteroepitaxially grow diamond on a silicon substrate.
Therefore, various kinds of studies has progressed and there is a report that it is effective that Pt or Ir is formed as a ground film, and then a diamond film is formed thereon by CVD method (see, for example, Y. Shintani, J. Mater. Res. 11, 2955 (1996), and K. Ohtsuka, Jpn. J. Appl. Phys. 35, L1072 (1996)).
In the current situation, the research relating to Ir has particularly progressed most. There is a method in which, first, by using a single crystal MgO as a substrate, an Ir film is heteroepitaxially grown thereon, and next by DC plasma method, the Ir film surface is pretreated by ion irradiation with a methane gas diluted by hydrogen, and a diamond film is grown on the Ir film. Thereby, there have been obtained diamonds having a conventional submicron size to a recent several millimeters' size.
However, linear expansion coefficients of the MgO substrate, the Ir film, and the diamond film are 13.8×10−6 K−1, 7.1×10−6 K−1, and 1.1×10−6 K−1, respectively. The difference of them is large. Therefore, there has been caused delamination at the respective interfaces between the MgO substrate and the Ir film and between the Ir film and the diamond film. In the present situation, it is being studied to form a diamond film having a large area of a 10-mm-square size or a 25-mm-diameter size. However, there is a problem that a film having even a 10-mm-square size does not become a continuous film, and only, films having about several millimeters size weakly adhere in a discontinuous manner or the film is occasionally delaminated in about several millimeters' pieces.